Mode I fracture toughness, KIc, is a key measure of rock strength. In volcanology, KIcis particularly relevant for dike propagation, as it quantifies the critical stress intensity factor required for fracture propagation under tensile stress. KIc has been extensively studied for rock types and construction materials relevant to civil engineering, mining, and hydrocarbon-related applications. However, there is a paucity of data for volcanic rock. In this study, we present KIcvalues for basalt samples of varying porosity and texture from four different lava flows on Mt. Etna (Italy) and investigate the influence of key microstructural parameters on KIc.We conducted 12 mode I fracture toughness experiments under dry, ambient pressure conditions on Cracked Chevron-notched Brazilian Disc specimens and determined KIc using a standardised method. Additionally, we characterised rock physical properties including porosity, elastic wave velocities, and permeability, and analysed thin sections to determine mineralogical composition and rock texture. We compared the physical and microstructural properties of the four lavas and then assessed those properties regarding any correlations with KIc.The fracture toughnessmeasurements were successful for 10 of the 12 specimens, yielding KIcvalues of 0.6–1.3 MPa·m1/2. Average connected porosity varied between 9­ and 17%. P-wave velocities varied from 3.1 to 3.8 km/s, while permeability varied from 6.7·10-17 to 6.3·10-12 m2.Our fracture toughness data are consistent with experimental data from the literature, fitting the general trend of high KIc typically corresponding to low porosity. However, within our small data set of rather heterogenous porosity characteristics and rock textures, we observe no strict inverse correlation of KIc,and porosity, since the porosity range of our samples is only moderate and other microstructural factors (e.g. pore size and shape) can dominate fracture behaviour in individual cases. We observe no systematic relationship between elastic wave velocities and KIc.